1) No clear (roles ,responsibility & business) definition
2) Not knowing clear strength of the partner
3) No idea of how team can produce results which individuals can not
4) Different (Goals & Values)
5) No (large heart & attitude of leg go)
6) Attitude, ego and I (don't need ne one / I alone can do it) mind set
7) Mixing personal problems with business issues
8) Joining hands (without due diligence or for limited selfish motives)
9) Unable to accept one as a leader
Ambanis, Bajajs, Goenkas, Birlas, Mafatlals, Microsoft Partners, Salim Javed, Thackreys are examples of many many many such broken partnerships in business, politics and families.
Mostly it destroys value, Its sad , its worthless & repenting in the long-run. Synergy is in joining hands & reverse of synergy is in separation.
I invite readers with many more such insights for the benefits of other readers.